1. Field of the Invention
The present invention relates generally to pulse synthesizers and more particularly to such synthesizers for use in processing data present in analog form on a magnetic tape.
2. Description of the Prior Art
In the recording of analog data onto magnetic tapes it is a common practice to record a digitizing reference signal with highly stable frequency at the same time the analog data is recorded. On playback, this reference signal is utilized to synchronize an analog to digital converter. Since the reference signal is subjected to many of the same variations in tape speed that the data is subjected to, the effects of these variations will cancel out in the digital data.
One short coming of this method is that analog tape recordings are subject to amplitude drop out due to imperfections in the magnetic material present on the tape. In addition, for various reasons, extraneous noise spikes appear on reproduced analog tape data. Thus, if a drop out or an extraneous noise spike appears on the digitizing reference signal it can produce a large error in the time base of the digital data. What is needed is a method of obtaining the correct average number of digitizing pulses from the analog tape reference signal.
One prior art method of obtaining the correct average number of digitizing pulses from an analog tape reference signal consists of tracking the average frequency of the reference signal with a phase-lock-oscillator and then utilizing the output of the phase-lock-oscillator to synchronize the A-D converter. The major disadvantage of this method is that fast phase fluctuations in the reference signal are not followed.
Another prior art method which has been utilized consists of following the average reference signal frequency with a phase-lock-oscillator but supplying the analog-to-digital synchronization pulses directly from the reference signal itself. In this method, if a pulse from a reference signal fails to appear within a specified time interval after it is expected, pulses are synthesized from the output of the phase-lock-oscillator. As soon as the pulses are again received directly from the reference signal, the phase-lock-oscillator output is suppressed. This method supplies the proper average number of synchronization pulses for the case of drop out but it has no provision for suppressing extra pulses due to extraneous zero crossings of the reference signal caused by noise being present upon the reference signal.